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High doses of the drug methylphenidate — the active ingredient in Ritalin and other ADHD medications — reduced testes size and delayed descent of the testes in male monkeys younger than 5 years old, compared with monkeys not given the drug. The drug also reduced their testosterone levels at both low and high doses.

However, the effects were not permanent. Around age 5, both the treated and untreated groups reached similar stages of puberty.

The results are the first to suggest this drug may affect testes development and testosterone levels.

"This was an unexpected finding," said study researcher Dr. Donald Mattison, of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The original premise of the study was to examine whether methylphenidate damaged DNA, Mattison said.

The findings are very preliminary, and should be interpreted with caution, Mattison said. Additional studies should attempt to replicate the results.

Parents of children who take this drug for ADHD should talk with their child's doctor before making any changes to their son or daughter's medication, Mattison said.

Methylphenidate hydrochloride is also the active ingredient in the ADHD medications Concerta, Metadate and Methylin, according to the National Institutes of Health.

Mattison and colleagues administered either a low dose or a high dose of the drug to rhesus macaques. The low dose produced blood concentrations of the drug about equivalent to those of children who take the medication; the high dose produced blood levels that were five to 10 times higher. The control group of monkeys received only the material that the drug was dissolved in, but not the drug itself.

The monkeys were two years old at the study's start.

After 15, 19 and 27 months of treatment, monkeys treated with high doses of the drug had smaller testicles than the control group. Fewer animals in the high dose group had descended testes at 16 and 29 months after treatment.

Differences between the groups disappeared by the end of the study, when the animals were about 5 years old or older.

Should we be worried?

Previous studies have found ADHD medications may interfere with children's growth.

The new study is "another piece of evidence that this medication is a powerful medication that does something to the physiology," in this case, it delayed puberty, said James Swanson, director of the Child Development Center, at the University of California, Irvine, who studies ADHD in children but was not involved in the new study.

It's too soon to say whether the same effects could be seen in humans, Swanson said, so it's too soon to know whether the findings are concerning. The new work will hopefully provide direction for future research, Swanson said. Additional studies need to be conducted looking at the long-term effects of methylphenidate on children, Swanson said.